A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example, the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometers are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
A test structure on a substrate is often measured using the above apparatus in order to test the accuracy of a lithographic process. Following the lithographic process, an inspection tool is used that comprises at least in part a scatterometer as described above. The test structure may be an array of bars that form a grating. Alternatively, the patterned product itself may be inspected during the production of the plurality of product layers. The inspection tool may then be used in-line to ensure that each layer or each set of layers is meeting quality specifications. The features of the patterned product that are determined using the inspection tool may be: overlay (i.e. the extent to which subsequent layers of patterned product “line up”) and size and shape parameters of the patterns such as critical dimension (CD), height of a patterned product in a single or multiple layers, and sidewall angle (which is the angle between the layer surface and the rising side of the product).
United States Patent Application Publication No. 2004/0169859 A1 (Smith) describes a method and apparatus for scatterometry measurements as described above. It describes the use of libraries of predicted measurements and specifically, it describes a way a theoretical model of a structure is parameterized to allow the characteristics of the structure to be independently measured. The parameters of the physical structure are varied over a predetermined range and the theoretical spectral result for each variation to the physical structure is calculated to define a library of spectra for the variable structure. Then, when (spectral) measurements of a presently measured physical structure are obtained, the library is searched to find the best fit. The characteristics of the presently measured physical structure may then be determined as closely as possible within the tolerances allowed by the results in the library. Alternatively, the library data may be used as a starting point and an estimation of an interpolation algorithm is used to refine the results such that a closest fit to the presently detected physical structure is found.
Untied States Patent Application Publication No. 2004/0169859 further describes a way of increasing measurement sensitivity (and possibly permitting measurements with higher spatial resolution) by introducing the periodical excitation of the physical structure with an intensity modulated pump beam. A second probe beam is directed to overlap at least a portion of the area which is being periodically excited. The combination of the modulated beam and the probe beam then gives an output at a detector that can be processed in a manner to extract the modulated portion of the signal the features. Specifically, a narrow wavelength beam from a laser that can be focused on a smaller spot may be used in combination with a broader polychromatic beam, thereby allowing a small spot measurement but over a range of wavelengths.
This method is known as modulated scatterometry. Modulated scatterometry is a pump-probe concept: a pump beam excites the substrate and the probe beam measures the change in a substrate property that is induced by the pump beam. A change in property can be very diverse. In its simplest form it is, for example, a reflectance variation. However, pump beams tend to alter a property of the substrate and are not useful for in-line measurement.
Nothing has been found in the prior art that discloses how to ensure that the correct portion of the physical structure on a substrate is being detected. In other words, although a measurement beam's spot size may be reduced, the accuracy of its position relative to a pattern on the substrate does not increase correspondingly. This accuracy may be particularly important if the physical structure is a pattern that is not periodic. Measurements from the wrong area of a non-periodic structure or pattern may give rise to a diffraction spectrum that may not necessarily match diffraction spectra existing in predetermined libraries. Errors may thereby be detected that are not, in fact, errors in the printing of the structure, but may be merely errors in alignment of the measurement beam. Alternatively, if the measured spectrum does match a stored spectrum but the position of the measured portion is incorrect, errors in the printing of the pattern may go unnoticed.